Soul Stone farming

Already tried searching the forums (search feature), but all I get are pages for people buying/selling/begging for soul stones. I was wondering which is the (if there is any) best training to chance upon soul stones. One would thing the lower episodes that cost less would be best, but only if the chances are the same for all episodes.

Do we know if other episodes grant higher chances for SSS? I still feel very new to this game with a lot of things, so please forgive my ignorance

It is believed that the odds of getting a soul stone is weighted against TP spent. In short this means that you can do whatever training episode you like and you should end up with approximately the same amount of soul stones for TP spent.